1. Field of the Invention
The present invention generally relates to devices used to mark and locate obscured objects, and more particularly to an electronic marker and template used to locate features of a buried access hatch such as a manhole or utility hole cover.
2. Description of the Related Art
Buried conduits are employed for supplying a wide variety of utilities, including pipelines for gas, water and sewage, and cables for telephone, optical fiber, power and television. It is often necessary to locate manhole covers, which are used for entrance or access to these conduits or vaults, in order to repair, modify, or replace the hatch or cover. Oftentimes when laying out a new road, a pavement crew will pave over a manhole cover with asphalt or concrete, and a different crew will come later to remove the material overlying the manhole cover so it can be raised or otherwise aligned with the street level. Conversely, it is important to know with as much accuracy as possible the approximate vicinity of such buried objects in order to avoid disturbing them when digging or excavating for other purposes. Above-ground marking devices may be installed immediately after the manhole cover is buried, but it is also common to use underground marking devices or systems to enable the later location of a utility access hatch.
In the past, different approaches have been used to indicate the presence of buried access hatches, namely, warning tape, painted symbols or surface markings, and electronic marker systems.
A warning tape is simply a band of plastic which is placed above the manhole cover before burial. These tapes are used to alert the excavation team of the presence of the manhole before any damage thereto might occur. As the backhoe or other mechanical digger excavates the site, it will hopefully uproot a portion of the warning tape prior to contact with the manhole.
Painting symbols on the ground is another common method of indicating the location of buried utility components. A worker can simply paint markings at the area above or near the buried component, and at a later time (using visual identification) pinpoint the location previously marked. Other types of visual surface markings can be used in place of painted markings, such as adhesive markers attached to the surface, or small metallic markers embedded into the ground so as to be level with the surface.
Electronic marker systems for locating buried objects are known in the art, and generally consist of two types, namely, active and passive markers (transponders). Active markers require the use of a power supply which amplifies a signal source (usually an AC signal). The signal is radiated by the underground marker and detected by a receiver unit above ground. Active markers may be “smart”, that is, contain encoded data which is transmitted with the locator beacon. Passive markers, in contrast, have no power supply, but rather operate in a resonant mode, responsive to a transmitted electromagnetic field, and are usually limited to detection of a single frequency signal.
A passive marker is basically a wire coil and capacitor surrounded in a protective envelope, which is then buried adjacent to the cable, pipe, or other object to be located. The marker is self-contained, with no external, accessible connections. Passive markers are activated by radiating a signal into the ground in the area where the marker is expected to be found. The signal is emitted via an inductive coil held close to the surface (the transmitter portion of a transceiver). When the coil is directly over, or near, the passive marker (which is itself an inductive coil), the marker accepts energy within its bandpass and stores it, reaching a sustained amplitude during the transmission cycle. When the transmission cycle ends, the marker re-emits the energy at the marker's resonant frequency with an exponentially decaying amplitude. A second coil within the transceiver unit acts as a receiving antenna which detects the re-radiated energy, alerting the locating technician with an audible tone or other indicator means.
There are several kinds of passive transponders for different applications. These include small, near-surface markers for locating objects just inches below the surface, medium size or mid-range markers, full-range markers for locating more deeply buried objects, and self-aligning markers such as the so-called ball marker which supports the marker coil horizontally, regardless of the orientation of the housing. There are hybrid systems wherein, for example, a signal is applied to a buried conductor (cable or trace wire), and coupled through the conductor to one or more markers buried adjacent the conductor. Also, a marker can be used to couple one conductor to another, so that the test signal may be conveyed to the second conductor without a direct physical connection. All of these markers generally float around the underground feature in the soil, and are subject to soil movement.